Surface measurements of the flow from a well have generally proved inadequate to effectively evaluate the downhole production and/or injection flow profiles of that well. To obtain the more accurate measurements needed for these profiles, several different types of tools have been developed which can be lowered into the wellbore for making measurements of certain downhole flow parameters, e.g. velocities, at various depths in the well. Two of the better known of these tools are the "continuous spinner flowmeter" and the "diverting or basket flowmeter". For a good general description of these types of flowmeters, see "A New Flowmeter for Production Logging and Well Testing", G. E. Piers et al, SPE 16819, 62nd Annual Tech Conf. of Society of Petroleum Engineers, Dallas, TX, Sept. 27-30, 1987 and "Production Logging-The Key to Optimum Well Performance", R. T. Wade et al, JOURNAL OF PETROLEUM TECHNOLOGY, Feb., 1965, pps.137-164.
In both types of flowmeters mentioned above, the flow velocity is determined from the rate of rotation of a propeller-type impeller (spinner) which is positioned in and powered by the flow stream. In the continuous spinner flowmeter (both conventional and full-bore), the impeller or spinner is situated at the lower end of the logging tool where it is exposed directly to the full flow stream. Inhomogeneities in the flow patterns for multi-phase flow, especially in wells deviated more than about 8.degree., render data from a continuous spinner flowmeter extremely difficult if not impossible to interpret even though such conditions are common. For example, in two-phase flow in a deviated well, downflow often occurs along the lower portion of the well, even though the net flow of both phases is up. Under these conditions, a spinner positioned along the lower side of the wellbore, or sometimes even if centered, may indicate flow in a direction opposite to that which is actually occurring.
Diverting flowmeters of the packer or basket types are designed to alleviate the problems encountered by the continuous spinner flowmeters by channeling the flow to an impeller which is positioned within the housing of the logging tool. At the higher flow velocities resulting from the diversion of the entire wellbore flow through a relatively small passage in the tool housing, severe flow inhomogeneities do not develop as readily and the revolutions (RPMs) of the impeller are a far superior indication of total flow velocity.
Unfortunately, however, diverting flowmeters suffer two significant limitations. Unlike the continuous spinner flowmeters, diverting flowmeters must be stationary in the wellbore while a reading or measurement is made. Thus, flow readings are obtained only at discrete depths in the well, making the precise location of fluid entries or exits more difficult to resolve within a reasonable logging time span, as compared to the results from a continuous logging measurement. In addition, diverting flowmeters are limited as to the maximum flow rate that may be measured. At high flow rates, the pressure drop associated with the flow of the wellbore fluid through the narrow passage in the housing of the logging tool is sufficient to force the tool from its desired vertical position within the well. For example, the basket flowmeter normally can not be used in wells having flows greater than about 3500 barrels per day.
From the above, it can be seen that a need exists for a wellbore flowmeter which can operate in a continuous mode in wells having high flow rates as does the present continuous spinner flowmeters while at the same time, achieving an accuracy comparable to that of a diverting-type flowmeter.